Filter screens for use in such machines are typically constructed from woven wire cloth. When weaving wire cloth the warp wires are those that run along the length of the roll of wire-cloth as it is woven, and wound around the take-up drum, while the weft wires are those which run across the width of the wire-cloth.
Square mesh wire cloth is comprised of nominally identical numbers of warp and weft wires per unit area, and a common wire diameter. For example, a 200# market grade cloth has 200 warp wires per inch and 200 weft wires per inch. Both warp and weft wires are 0.050 mm in diameter.
Although there is still a desire to use wire cloth with generally square openings, and the use of generally square mesh as the filter media for oilfield screens is still widespread, rectangular meshes have proved to be successful as a robust, high capacity alternative to square mesh.
Robust filter media incorporating rectangular mesh are disclosed in U.S. Pat. No. 5,944,197 and PCT Application PCT/GB2002/005018.
A rectangular mesh is normally woven with more warp wires per unit length than weft wires per unit length, since the time taken to weave a given length of wirecloth is dependent on the number of weft wires.
One common type of screen comprises layers of mesh bonded to a support structure (normally referred to as a frame) which is usually generally flat and rectangular in shape, and which contains a number of similarly sized (normally rectangular) openings across which the screen mesh is tensioned. The mesh is supported by the frame and the openings in the frame define a corresponding number of mesh covered windows for filtering the fluid materials. The frame may be of metal but more preferably is of a plastics material particularly GRP and preferably is reinforced internally by a wire or rod framework. Such screens will be referred to as integral screens, that is the mesh and frame are integrated by the bonding of the mesh to the support frame. A jig for making integral screens in which two screens are made at the same time, is described in GB Patent Specification 2,382,037. Such a jig will be referred to as a jig of the type described.
In operation the maximum stress on the wire cloth in such a screen is found to occur at the middle of the longer dimension of the frame. This suggests that the wires running parallel to the shorter sides are subject to greater stress than those running parallel to the longer sides of the screen. Areas of maximum stress are indicated in FIG. 1, which is described more fully later.
It has also been observed in practice that the wires running parallel to the shorter span of the mesh in such a frame often tend to fail first, which also supports the theory that these wires are subject to greater stress.
Another common type of screen is a so-called hook-strip screen. Such a screen consists of generally rectangular sheets of wire cloth (mesh) with hooks along two parallel sides. The sheets are attached by the hooks to a stretching mechanism in the shaker. This stretches the mesh to tension the wire cloth. This is necessary to encourage good solids conveyance across the stretched mesh in use.
In practice hook-strip screens are usually stretched over a support which presents a convex upper surface to the mesh so that the mesh in tension becomes convexly curved as shown in FIG. 2. In general only two edges of the mesh include hooks, and the other two edges are not secured to the shaker. Therefore the tensioning load is applied in one direction only. This means that if the screen is over-tensioned the wires parallel to the tensioning direction will tend to fail before the wires extending in the perpendicular sense. In use, over-tensioning can occur due to excessive solids build-up or any general overloading of the screen, as well as due to any inappropriate tensioning of the mesh during set-up.